In many integrated circuit designs it can be desirable to provide a reference circuit. A reference circuit can provide a current and/or voltage at a generally known value. Reference circuits can have numerous applications, including but not limited to establishing a reference voltage to detect input signal levels, establishing a lower supply voltage to some section of a larger integrated circuit (e.g., memory cell array), establishing a reference voltage/current to determine the logic value stored in a memory cell, or establishing a threshold voltage for some other functions.
Reference circuits can be non-biased or self-biased. Non-biased reference circuits can rely on discrete voltage drop devices to arrive at a reference level. For example, a non-biased reference circuit can include resistor-diode (or diode connected transistor) arranged in series between a high supply voltage and a low supply voltage. A drawback to such approaches can be that a current drawn can be proportional to supply voltage. Thus, a higher supply voltage can result in a higher device current (ICC). This can be undesirable for low power applications.
Self-biased reference circuits can rely on transistor biasing to provide a reference current that is less variable (or essentially not variable) in response to changes in power supply voltage.
To better understand various features of the present invention, a conventional self-biased reference circuit with corresponding start-up circuitry will now be described.
FIG. 5 shows a conventional resistor-transistor divider circuit 500. Conventional resistor-transistor divider circuit 500 can receive power via a power supply voltage (Vcch) and a ground voltage (Vgnd). A resistor-transistor divider circuit 500 can include a first current mirror formed by p-channel metal-oxide-semiconductor (PMOS) transistors P51 and P52 and a second current mirror formed by n-channel MOS (NMOS) transistors N51 and N52. A bias point for such current mirrors can be established by a resistor R51.
Bias voltages Vbiasp1 and Vbiasn1 can be provided at gate-gate connections of transistors P51/P52 and N51/N52, respectively. An additional bias voltage Vbiasn2 can be generated by diode configured transistors N53 and N54 connected in series between a drain transistor P52 and ground voltage Vgnd. Similarly, an additional bias voltage Vbiasp2 can be generated by diode connected transistors P53 and P54 connected in series between a power supply voltage Vcch and the source of a transistor N55. Transistor N55 can be biased with voltage Vbiasn1.
Some or all of bias voltages Vbiasn1, Vbiasn2, VbiasP1 and VbiasP2 can be provided as input voltages for transistors (i.e., connected in a cascode fashion) in other analog circuit blocks. Such circuits can include current reference circuits, voltage reference circuits (including “band-gap” reference circuits), voltage regulator circuits, and low voltage detect circuits.
In addition to providing reference voltages at know levels, resistor-transistor divider circuit 500 can protect such other circuits from high voltage levels by acting as a buffer with respect to a high supply voltage Vcch.
A drawback to a conventional circuit like that shown in FIG. 5 can be undesirable variation in the bias voltages provided. In particular, a resistor-transistor divider circuit current can vary across operating and manufacturing conditions (i.e., process variations, operating voltage variations, and/or temperature variations (PVTs)).
Another drawback to a conventional circuit like that of FIG. 5 can be lack of flexibility and large circuit components needed for implementation. In particular, it can be difficult to optimize bias signals while at the same time providing the ability to handle a wide range of device power supply voltages (e.g., 1.6 V to 6.0 V). Further, to arrive at a small reference current Iref, a relatively large resistor R51 is needed.
Still further, in some cases a resistor-transistor divider circuit 500 may require special or additional protection transistors to be included as increased power supplies are used. In one conventional case, high voltage transistors can be formed with specialized manufacturing steps. A drawback to this approach is the added complexity to the manufacturing process. Adding transistors to accommodate a wide rage of power supply voltages can require a metal option and/or bond option to include/exclude such additional transistors as needed. This undesirably adds another manufacturing step to a device, increasing costs.
It would be desirable to arrive at a self-biased reference circuit that can operate at a wider range of power supply voltages without the drawback of the above conventional approaches.
It would also be desirable to arrive at a self-biased reference circuit that can operate at low current levels and yet not require large resistors.
It would also be desirable to arrive at a self-biased reference circuit that can buffer reference circuits from higher supply levels that does not include high voltage transistors.